Automobile blade type plug-in fuse assemblies commonly comprise a two-piece assembly heretofore having a thin, box-like housing and a planar, plate-like, all metal plug-in fuse element secured therein. The metal plug-in fuse element has a pair of spaced, confronting, exposed terminal blades extending from one end of the housing. Current-carrying extensions of the terminal blades extend into the housing where they are closely encompassed by the housing walls. A fuse link unsupported between the ends thereof extends suspended between the current-carrying extensions and is spaced from the housing side walls which are closely spaced from the fuse link in comparison to, for example, the spacing of the housing walls from a fuse link of conventional threaded type fuses used in homes.
The housing has slot-like spaces opening to one end of the housing, the terminal blades project from these spaces where they can be plugged into socket clips in a mounting panel or fuse block. This structure and method of making the same are described in other patents including U.S. Pat. Nos. 3,909,767 and 4,344,060. Fuse blocks in presently made automobiles generally provide a vertical mounting wall designed so that when the fuse is mounted thereon the terminal blades extend horizontally in a horizontal plane from the housing and gravity acts on the fuse link in a direction where any significant sagging thereof can cause the fuse link to touch the housing side wall spaced a short distance below it and cause problems to be described.
The fuse link of this and other types of fuse assemblies melts and sometimes vaporizes under fuse blowing conditions. Fuses generally are designed to blow under both prolonged modest overload current (like 135% of rated current within 1/2 hour) or instantly under short circuit current. Under such a prolonged modest overload current the temperature of the fuse link progressively rises until the fuse blows. The temperature rise in the fuse link results from electrical power dissipation in the electrical resistance R of the fuse link material due to electrical current I flow therethrough. The formula describing this power dissipation P is P=I.sup.2 R.
Under normal operation (normal current is usually about 80 percent of rated current), the heat dissipated in the fuse link is sufficiently small that a large section of the fuse link does not melt or even soften. Heat generated in the fuse link is conducted into the terminal blade portions, housing and panel socket clips. When a current substantially above rated current (like 135% thereof) flows in the link, the heat dissipation is such that there is an insufficient rate of conduction of heat therefrom so that the temperature rises to the blowing temperature. The fuse link will soften before it melts, and it is important to the reliability of the fuse that before blowing the fuse link does not soften to a degree that a large section of the fuse link sags against a side wall of the housing before the desired fuse blowing conditions occur. If this occurs, the contact made between a sagging fuse link and the housing can melt the fuse housing and cool the fuse link and prevent it from blowing in the desired time period or from blowing at all. Such contact, in any event, modifies the blowing characteristics of the fuse link. The automobile blade fuses have heretofore been made only for current ratings up to 30 amperes. The fuse links of these fuses are so short and thin that they do not generally sag enough to cause any problems. However, such fuses are now being designed for currents well above 30 amperes and their fuse links are so long and thick that, prior to the present invention, a serious sagging problem was encountered if not supported.
A fuse link of even cross-sectional areas throughout will generally have the hottest spot at the center thereof. In such case, the temperature verses fuse link position curve progressively increases toward the center point thereof. In designing a fuse for a given blowing delay time, if it is necessary to increase the delay from an initial test value, the volume of the fuse is increased by increasing the length and/or cross sectional area thereof. The latter decreases the overall fuse link resistance while the former increases its resistance. The increase in fuse length also increases the tendency of the fuse link to sag.
Sometimes the desired fuse parameters are achieved by providing a central fuse link of suddenly reduced cross section which provides a temperature verses fuse link position curve which sharply rises at the center of the fuse link. In either case the fuse link will usually initially blow at this center point.
The sagging problem described does not occur if the fuse housing is filled with a material like sand, which is used in some fuses to quench high energy arcs. The use of sand, however, purely as a support for a fuse link is not practical in fuses where arc quenching can be achieved in a more economical way. Thus, the packing of sand or other support materials into the fuse assembly housing requires an additional assembly steps and material that add to the cost of the final product and is therefore undesirable.
The blowing current of the fuse link is a function of many factors including the resistance of the fuse link and the metal alloy out of which it is made, as well as the configuration thereof. It is known that the blowing current or blowing temperature of a fuse link can be reduced by applying a material such as tin to a fuse link or other fuse configuration, like a spiral winding of fuse wire on a core of insulating material. Thus, in prior art fuses tin has been applied in the form of beads on the successive windings of a spiral wound fuse wire to reduce the blowing current or blowing temperature of the fuse wire. In a fuse link used in a conventional threaded type fuse used in homes, tin has been applied for this purpose in the form of a plug overlay or globule of tin added to the fuse link on only one side of the reduced center portion of the fuse link involved. The tin migrates at a rate which is a direct function of the temperature of the point on the fuse link toward which the tin can migrate. In the threaded fuse described, the tin migrates at the highest rate toward the hottest center portion of the fuse link. The tin alloys with the fuse metal and reduces the blowing current.